The primary reason Truman altered the policy previously followed by Roosevelt is because of the aggression displayed following the surrender of Germany. The U.S. and the Soviets had not been comfortable allies; in fact they fought on the same side on opposite ends of the conflict, the Soviets from the East and the U.S., Britain and France from the West. It is not known of Roosevelt would have modified his policy had he lived; although Winston Churchill believed that Roosevelt was naive and too trustful of Joseph Stalin at the Yalta Conference. At any rate, after the surrender of Germany, the Soviets set up puppet governments in East Germany, Poland, and Romania. Rather than liberating these nations, they dominated them. Additionally, they soon began agitating in Greece and Turkey to undermine legitimate governments there. It then appeared obvious that the Soviets were intent on dominating as much of Europe as possible. To that end, Truman issued the Truman Doctrine:

I believe that it must be the policy of the United States to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures.

Shortly thereafter, Truman's adviser, Bernard Baruch in an address to the South Carolina General Assembly said:

Let us not be deceived – today we are in the midst of a cold war.

From this speech, and the actions of the Soviets, the term "cold war" was born. Truman altered his policies because he had no choice.